Light-emitting diodes (LEDs) have been vigorously developed in recent years accompanied by the development of aluminum/indium/gallium/phosphorus (AlInGaP) that emits light in red to orange colors and gallium nitride (GaN) that emits light in a blue color as light-emitting materials. There have further been realized LEDs that emit near ultraviolet rays of shorter than 400 nm, such as 365 nm and 370 nm. An LED which emits white light has also been accomplished by combining, for example, a fluorescent material with a blue LED or a near ultraviolet LED.
The LED has many advantages such as a long life, a high temperature stability, easy dimming and a low driving voltage, and has positively been applied to displays, indicator boards, car-mounted illumination, signal lamps, cell phones, video cameras and the like. In particular, a white LED has been developed for illumination purposes and has been very expected as a source of light to substitute for the conventional incandescent lamps, halogen lamps and fluorescent lamps. For its widespread use, however, it is desired to further improve the brightness and efficiency as a source of light.
In these applications, the LED is usually used being packaged. In mounting the LED on a package, in general, the LED is joined to a predetermined position in the package while electrically connecting the electrodes of the package to the electrodes of the LED, and is sealed with a transparent sealant for protecting the LED. A widely used sealant can be represented by an epoxy resin using a bisphenol A-type glycidyl ether on account of its highly adhering property, good operability and low cost.
However, the above epoxy resin sealant cannot meet the shortened wavelengths and high brightness of the LED. When used for sealing the near ultraviolet LED and white LED, therefore, there arouse such problems that the resin is deteriorated and becomes yellow causing the brightness of the LED device to decrease and the color tone to be varied.
Studies have been forwarded to solve these problems without, however, finding any solution. For example, the light resistance can be improved to some extent by adding an alicyclic epoxy compound to a hydrogenated bisphenol A type glycidyl ether. However, the degree of improvement is far from the practicable level and the heat resistance is rather lowered (e.g., see patent document 1). Further, by using a curable adamantane compound which is an epoxy compound using adamantane as a basic skeleton, there can be obtained a resin with a heat resistance higher than that of the hydrogenated bisphenol A glycidyl ether, which, however, is not still of a sufficient level (see, for example, patent document 2). When a phosphorus type antioxidant is added, discoloration due to the heat can be suppressed to some extent causing, however, the light resistance to be deteriorated.
Patent Document 1: JP-A-2003-73452
Patent Document 2: JP-A-2005-146253